Photo-tailored heterocrystalline covalent organic framework membranes for organics separation

Organics separation for purifying and recycling environment-detrimental solvents is essential to sustainable chemical industries. Covalent organic framework (COF) membranes hold great promise in affording precise and fast organics separation. Nonetheless, how to well coordinate facile processing—high crystalline structure—high separation performance remains a critical issue and a grand challenge. Herein, we propose a concept of heterocrystalline membrane which comprises high-crystalline regions and low-crystalline regions. The heterocrystalline COF membranes are fabricated by a two-step procedure, i.e., dark reaction for the construction of high-crystalline regions followed by photo reaction for the construction of low-crystalline regions, thus linking the high-crystalline regions tightly and flexibly, blocking the defect in high-crystalline regions. Accordingly, the COF membrane exhibits sharp molecular sieving properties with high organic solvent permeance up to 44-times higher than the state-of-the-art membranes.


4.
Chemical structure of COF membrane should be fully characterized, such as NMR measuremernt. 5. What is the advance of the proposed strategy compared to other ways of compensating for defects, such as secondary interfacial polymerization? The authors should comment on that. 6. Can you please explain why the initial ethanol contact angle of the different COMs is similar, but other initial solvent (acetonitrile, ethyl acetate, methanol et al.) contact angle is different? 7. The COF film is physically transferred onto the support. I assume that delamination/detach of this COF layer may occur, the authors should comment on the adherence of the COF layer to the support.
8. An important issue is the stability of the COF materials. Did the authors study the effect of water and different pH on their COF membranes?
Reviewer #3: Remarks to the Author: In this manuscript, the authors propose a photo-tailoring strategy to fabricate the heterocrystalline COF membranes (COM) with both high-crystalline regions (dark reaction) and low-crystalline regions (photo reaction). It was demonstrated that post-introduction of low-crystalline regions via photoinduced reaction can repair the defects from high-crystalline regions, thereby promoting co-existence of both regions. The resulting COM displays sharp molecular sieving properties with remarkable organic solvents permeance up to 44-times higher than the state-of-the-art membranes. The manuscript is well organized and presented clearly. These findings are interesting and provide new insights in the design of heterocrystalline COM. Considering the overall high quality of this work, I recommend its acceptance by Nature Communication after some minor revisions. The specific comments are as follows.
-The synthesis of imine-linked COF membranes via interfacial polymerization is affected by many factors such as monomer concentration, reaction time, catalysis, etc. While the authors control the same conditions for COM synthesis, it is better to present the key details such as concentration and dark reaction time in the manuscript. In addition, dark reaction time is of importance for synthesis of high-quality heterocrystalline COFM. It is suggested to demonstrate such point in the manuscript.
-The concentration of Tp and Bpy is separately fixed at 0.02 mmol and 0.03 mmol for the synthesis. How did the authors make sure that such concentration is optimum parameter for COM performance based on two-step reaction? -In Figure 3a, the insert cross-sectional images are too small to read; please move them to SI or increase the size of these figures.
-During a dark reaction process, the enol-imine linkage would tautomerize irreversibly to stable ketoenamine form. Which curve is the spectrum of DCOM in Figure S2? Why is the intensity of C=C band very weak? If possible, XPS analysis is required to verify the presence of keto-enamine-linked COM.
-The increase of an irradiation intensity lead to a marked increase of C=C band compared to dark reaction. Please provide a detailed explanation.
-It was found that more rapid reaction rate corresponds to a marked increased of C=C band and more low-crystalline regions. Are there any links between them? -Is that possible to reverse the sequence of dark reaction and photo reaction for tuning the crystallinity and performance of COMs.
-I am curious about the permeance of n-hexane for DP2hCOM. In this work, the authors propose a concept of heterocrystalline membrane, using covalent organic framework membrane (COM) as model membrane to solve the dilemma between high crystallinity and easy fabrication of defect-free COM. The heterocrystalline membrane is formed using a two-stage process and photochemistry to generate lowcrystalline COF regions that act to block the defect in a high-crystalline COF system. The resulting membranes are utilized for the separation of organic solvents and achieve the ever-reported highest selectivity and solvent permeances. This work is of interest for the broad community of COF material science, membrane science, and organics separation.
Therefore, I would like to suggest accepting this paper after addressing the following minor issues.

Reply:
Thank the reviewer for the highly positive remarks and strong support of publication of this work.
1. The authors claim that this photo-tailored crystallinity strategy is practical to Schiffbase COM with a very nice demonstration. I wonder whether the photo irradiation will influence the crystallinity of other kinds of Schiff-base COFs.

Reply:
Based on the reviewer's valuable guidance, we further prepared another two kinds of Schiff-base COMs (Tp-Tta and Tp-Azo) under either dark condition or photo-condition 2 with irradiation intensity of 9.0 mW/cm -2 . The crystallinity of COMs was evaluated by Xray diffractometer (XRD). As illustrated in Fig. R1, the Tp-Tta and Tp-Azo COM formed under dark condition show an intense and sharp peak at ~5.8° and ~3.2°, respectively, corresponding to reflections from the (100) lattice plane. By contrast, the Tp-Tta COM and Tp-Azo COM formed under photo-condition display a substantially weaker and wider (100) diffraction peak, indicating the pronounced influence of photo-irradiation on the COM crystallization. The relevant discussions have been added in the revised manuscript and revised supplementary information.

The manuscript was revised as follows:
▪ Section of photo-tailored reactive-crystallization of Schiff-base COM, Paragraph 2 "These findings demonstrate a simple and effective strategy for tailoring the crystalline structure of Tp-Byp COM. To evaluate the generality of this strategy, we further prepared two kinds of Schiff-base COM, Tp-Tta and Tp-Azo. It has been found that both the Tp-Tta and Tp-Azo COM formed by photo reaction exhibit notably less crystallinity than those formed by dark reaction (Supplementary Fig. 7). This strategy offers the possibility to tailor heterocrystalline COM by controlling the dark and photo 3 reactions during membrane formation." The supplementary information was revised as follows: 2. The authors evaluate the separation performance at a concentration of 50 ppm. Since the organic concentrations in the industry are usually higher than 50 ppm, even ten-fold higher in some cases, so how about the separation performance of the membrane for higher concentration organic solutions?

Reply:
Thanks for the reviewer's valuable guidance. We further evaluated the separation performance of the DP2hCOM for dye solutions with concentration ranging from 20 ppm to 500 ppm. As shown in Fig. R2, the DP2hCOM maintained high dye rejection (>95%) under varied feed concentrations. Additionally, the rejection slightly increases from 95% to 97% when dye concentration increases from 50 ppm to 500 ppm due to the larger dye clusters arising from the more favorable dye aggregation at higher dye concentrations 1,2 .
The relevant discussions have been added in the revised manuscript and revised supplementary information.  c, long-term operating stability of DP2hCOM evaluated by cross-flow unit. The inset digital photograph is the feed and permeate solution, respectively. Fig. 3d, the authors use the full width at half maximum of diffraction peak to explore whether photo irradiation influences the crystalline structure of the highcrystalline regions. Why not use the intensity of the diffraction peak?

Reply:
Thanks for the reviewer's valuable comments. The full width at half maximum of the diffraction peak can reflect the size of the crystalline domains based on Scherrer equation 3 . 6 (1) β is the line broadening at half of the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians. τ is the size of the ordered (crystalline) domains; K is a dimensionless shape factor, with a value close to unity. The shape factor varies with the actual shape of the crystallite. Herein we have considered the shape factor as unity for the ease of calculation. λ is the X-ray wavelength which has the value 1.5418 Å; θ is the Bragg angle (in degrees). Therefore, we use full width at half maximum of diffraction peak to assess the effect of photo irradiation on the high-crystalline regions of COF membranes. in their study? And particularly is the separation performance comparable with DP2hCOM?

Reply:
Thanks for the reviewer's valuable guidance. We further fabricated a thicker DCOM (120h) with thickness of ~130 nm by prolonging the dark reaction time to 120 h. As illustrated in Fig. R3, although the rejection could be improved, the fabricated membrane showed only moderate ethanol permeance of about 25 L•m -2 •h -1 •bar -1 , which was 69% less than that of DP2hCOM, arising from the significantly increased mass transfer resistance due to the increased thickness.

Recently, organic solvent nanofiltration has attracted intense interest from membrane
science. What is the major distinction between water treatment membranes and organic solvent treatment membranes?

Reply:
The major distinctions between water treatment membranes and organic solvent treatment membranes are summarized in Table 1. Overcoming permeability/selectivity trade-offs c. Reducing fouling by high molecular species such as dissolved organic matter d. Minimizing degradation in harsh organic solvents and develop robust regeneration protocols to enhance membrane lifetime 9 Reviewer #2 (Remarks to the Author): In this manuscript, a heterocrystalline COF membrane comprising high-crystalline regions and low-crystalline regions was prepared through a two-step procedure based on sequential Schiff-base reactions. The bond linkage can tautomerize under photo irradiation, form low-crystalline regions, and link high-crystalline regions to obtain a defect-free membrane. This strategy delicately solves the dilemma between high crystallinity and easy fabrication of defect-free membrane, providing a new design prototype to process crystalline polymer materials. From this point of view, this work is highly innovative. The experiments are thorough and the manuscript is clearly written.
Therefore, I recommend publication after a minor revision. There are a few comments for authors to consider,

Reply:
Thank the reviewer for the highly positive remarks and strong support of publication of this work. Figure S6, the defects on the DCOM gradually decrease with the prolongation of the synthesis time. If the defects of DCOM can be eliminated via prolonging the synthesis time without photo-irradiation?

Reply:
Thanks for the reviewer's valuable guidance. We investigated the structure and separation performance of DCOMs fabricated in dark reaction for longer time.   Figure 3d, the intensity of the (100) diffraction peak of DPCOM decreases after photo-irradiation, while the full width at half maximum of the peak remains 11 unchanged. Detailed discussion should be provided.

Reply:
Thanks for the reviewer's valuable comments. The full width at half maximum of the diffraction peak reflects the size of the crystalline domains based on Scherrer equation 3 , and the intensity of the diffraction peak reflects the relative crystallinity of membrane 12 . After photo reaction, the low-crystalline regions of DPCOM forms, while the high-crystalline regions remain unchanged. Thus, the full width at half maximum of the (100) diffraction peak remains unchanged, but its intensity decreases because of the increased proportion of low-crystalline regions (Fig. R6).

Reply:
Based on the reviewer's valuable guidance, we further characterized the chemical structure of COF membrane by 13 C cross-polarization magic angle spinning (CP-MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy. As illustrated in Fig. R8, the spectrum of DCOM exhibits resonances at 151.0 ppm and 107.6 ppm, corresponding to the enamine carbon and the α-enamine carbon, respectively, which is consistent with the reported β-ketoenamine-linked COF membrane 13,14 . The spectrum of PCOM shows the same resonances as that of DCOM, while the peaks are wider and less resolved, suggesting low structural orderness 15  The supplementary information was revised as follows: ▪ Section 1.4

Nuclear magnetic resonance spectroscopy (NMR)
Solid-state 13 C cross-polarization magic angle spinning (CP-MAS) NMR spectra were performed on a Varian infinity plus 300 NMR spectrometer under 12 kHz spinning rate. 14 The samples were dried in hot air oven at 60 °C for 12 h and diced into small pieces before NMR analysis. between various COMs is slightly higher than the difference in initial ethanol contact angle. 16 7. The COF film is physically transferred onto the support. I assume that delamination/detach of this COF layer may occur, the authors should comment on the adherence of the COF layer to the support.

Reply:
In this study, polyethylene terephthalate (PET) microfiltration membrane was chosen as the substrate because of their high flexibility and abundance of ester groups that could form hydrogen bonds with the secondary amine groups and terminal primary amine groups of COF layer (Fig. R9). Additionally, the morphology of the skin layer is another non-negligible factor, that is, the rougher surface with greater surface area usually leads to higher adhesion strength between the skin layer and the substrate 20 . As shown in Fig.   R10, the COF layer generated by interfacial polymerization bears a rough surface on the side towards aqueous phase. When the COF layer is transferred to the substrate, the rough side faces down, providing a much higher surface area for anchoring the substrate. Fig.   R11 shows the digital photos of DP2hCOM deposited on PET substrate remaining intact after folding. Furthermore, it is well-known that the adhesion property between the substrate and the skin layer is crucial in determining the stable performance of thin film composite membranes. Fig. R12 shows that the DP2hCOM deposited on PET substrate is capable of withstanding continuous cross-flow shear forces and maintaining separation performance during 48-hour operation. These results indicate that the COF layer adheres well to the substrate.

Reply:
Based on the reviewer's valuable guidance, we evaluated the pH stability of COF membrane in both alkaline and acid solutions, aqueous sodium hydroxide (NaOH) solution (pH 13) and hydrochloric acid (HCl) solution (pH 2), respectively. The membranes were immersed in the above alkaline or acid solutions at room temperature.
As shown in Fig. R13, the rejection of DP2.5hCOM decreases by only 5% after immersion in strong alkaline solution or acid solution for 72 h, exhibiting good stability under extreme pH conditions. and alkaline aqueous solution (blue). 19 Reviewer #3 (Remarks to the Author): In this manuscript, the authors propose a photo-tailoring strategy to fabricate the heterocrystalline COF membranes (COM) with both high-crystalline regions (dark reaction) and low-crystalline regions (photo reaction). It was demonstrated that postintroduction of low-crystalline regions via photo-induced reaction can repair the defects from high-crystalline regions, thereby promoting co-existence of both regions. The resulting COM displays sharp molecular sieving properties with remarkable organic solvents permeance up to 44-times higher than the state-of-the-art membranes. The manuscript is well organized and presented clearly. These findings are interesting and provide new insights in the design of heterocrystalline COM. Considering the overall high quality of this work, I recommend its acceptance by Nature Communication after some minor revisions. The specific comments are as follows.

Reply:
Thank the reviewer for the highly positive remarks and strong support of publication of this work.

The synthesis of imine-linked COF membranes via interfacial polymerization is
affected by many factors such as monomer concentration, reaction time, catalysis, etc.
While the authors control the same conditions for COM synthesis, it is better to present the key details such as concentration and dark reaction time in the manuscript. In addition, dark reaction time is of importance for synthesis of high-quality heterocrystalline COFM.
It is suggested to demonstrate such point in the manuscript.

Reply:
Before synthesizing heterocrystalline DPCOM, we first optimized the reaction time and monomer concentration to obtain a high-crystalline and ultrathin DCOM.  The manuscript was revised as follows: ▪ Section of Photo-tailored reactive-crystallization of Schiff-base COM, Paragraph 1 "The optimal reaction time was set at 96 hours, and the Bpy and Tp concentrations were set at 0.30 and 0.20 mmol L -1 , respectively ( Supplementary Fig. 2, 3)." The supplementary information was revised as follows:  Top-view SEM demonstrates the fibre-like crystal assembly morphology of highcrystalline DCOM and the inter-crystal defects are obviously observed when the membrane thickness decreased to below 100 nm. Further reducing the monomer 24 concentration, the membrane thickness remained around 50 nm, but the inter-crystal defects became more severe. Hence, Bpy concentration of 0.30 mmol L -1 and Tp concentration of 0.20 mmol L -1 was selected as the optimum concentration.
2. The concentration of Tp and Bpy is separately fixed at 0.02 mmol and 0.03 mmol for the synthesis. How did the authors make sure that such concentration is optimum parameter for COM performance based on two-step reaction?

Reply:
Thanks for the reviewer's valuable comments. In the reply to question 1 of this reviewer, we have explained why the concentrations of Tp and Bpy are separately fixed at 0.2 mmol L -1 and 0.3 mmol L -1 , or 0.02 mmol and 0.03 mmol of amount, respectively.
3. In Figure 3a, the insert cross-sectional images are too small to read; please move them to SI or increase the size of these figures.

Reply:
Based on the reviewer's valuable guidance, the cross-sectional images insert in Figure 3a were moved to SI.

The manuscript was revised as follows:
▪ Section of preparation and characterizations of DPCOMs, line 123-125 "Moreover, the thickness of the DP2hCOM does not increase (~55 nm), suggesting that the low-crystalline regions grow in the intercrystalline defects instead of along the thickness of the membrane (Supplementary Fig. 11)." ▪ Section of preparation and characterizations of DPCOMs, Fig. 3   4. During a dark reaction process, the enol-imine linkage would tautomerize irreversibly to stable keto-enamine form. Which curve is the spectrum of DCOM in Figure S2? Why is the intensity of C=C band very weak? If possible, XPS analysis is required to verify the presence of keto-enamine-linked COM.

Reply:
Thanks for the reviewer's valulable comments. The blue curve named "0 mW cm -1 " is the spectrum of DCOM. We have changed the name to "DCOM" to make it clearer.
During the dark reaction process, the enol-imine linkage would tautomerize irreversibly to stable keto-enamine form (CO-C=C-NH) until the initial amorphous network convert to crystalline framework 21 . By contrast, during a photo reaction process, the phototautomerization of the enol-imine linkage would proceed via a low barrier 27 transition state without the limitation of crystalline structure. Therefore, the keto-enamine linkage (CO-C=C-NH) of DCOM is less than that of PCOM, and the C=C stretching band (1566 cm -1 ) of keto-enamine linkage is weaker. The related discussion in the manuscript was revised.
Based on the reviewer's valuable guidance, we performed XPS to verify the presence of keto-enamine-linked COM. As shown in Fig. R16, the N 1s

The manuscript was revised as follows:
▪ Section of photo-tailored reactive-crystallization of Schiff-base COM, paragraph 1 "The synthetic routes of COMs by either dark reaction or photo reaction were illustrated in Fig. 2a. Initially, precursor trialdehyde (Tp) and diamine (Bpy) would 28 polymerize into an amorphous network via enol-imine linkage 25 . During dark reaction, the reversible enol-imine linkage breaks and reforms slowly, thus converting the initial amorphous network into the thermodynamically stable crystalline framework as a result of the "error-correcting" process 26 Fig. 6)." ▪ Section of photo-tailored reactive-crystallization of Schiff-base COM, paragraph 2 "Besides, the FTIR spectra demonstrate that the C=C stretching bands in the ketoenamine linkage of PCOMs was more intense than that of DCOM ( Supplementary Fig. 4), ascribing from the small energy barriers of enol-keto phototautomerization." The supplementary information was revised as follows: ▪ Section 1.4

X-ray photoelectron spectrometer (XPS)
XPS spectra were performed using a K-Alpha+ spectrometer (ThermoFisher Scientific) and an Al-Ka x-ray source under high vacuum (5×10 −8 Pa). All binding energies were calibrated using C1s peak from the adventitious carbon at 284.80 eV.   5. The increase of an irradiation intensity leads to a marked increase of C=C band compared to dark reaction. Please provide a detailed explanation.
Reply: 30 Thanks for the reviewer's valuable comments. The increase of the irradiation intensity leads to an increased number of absorbed photons per unit time 25   6. It was found that more rapid reaction rate corresponds to a marked increase of C=C band and more low-crystalline regions. Are there any links between them? 31

Reply:
Thanks for the reviewer's valuable comments. Rapid phototautomeric reaction can promote the tautomerization of enol-imine linkage, resulting in a marked increase in the keto-enamine form linkage (CO-C=C-NH) which contains C=C bond. The reversible enol-imine linkage could break and reform during the formation of COMs, allowing the initial mismatched amorphous structure to convert into a high-crystalline structure 26 .
Under photo condition, rapid phototautomeric reaction leads in less reversible enol-imine linkage and more irreversible keto-enamine form linkage containing C=C bond, preventing the "error-correcting" of structure and generating more low-crystalline regions.
7. Is that possible to reverse the sequence of dark reaction and photo reaction for tuning the crystallinity and performance of COMs.

Reply:
Thanks for the reviewer's valuable comments. We reversed the reaction sequence of DP2hCOM and prepared P2hDCOM by 2-hour photo reaction first and dark reaction after.
The crystallinity of COMs was measured by X-ray diffractometry (XRD). As shown in Fig. R17a, P2hDCOM displays a weak and wide (100) diffraction peak with full width at half maximum (FWHM) of 1.32°, indicating that its crystallinity is inferior to that of DP2hDCOM. We assume that this phenomenon is because the initially formed lowcrystalline crystals by photo reaction affect the subsequent morphology evolution of P2hDCOM 27-29 . The separation performance of P2hDCOMs is shown in Fig. R17b, which demonstrates a high ethanol permeance of 148 L m −2 h −1 bar −1 but only a moderate dye rejection of 95%. The rejection is less than that of DP2hCOM (99%). It is demonstrated that conducting dark reaction first and then photo reaction is more effective in eliminating the non-selective intercrystalline defects in COMs than the reverse procedure, thus leading to a higher rejection.

Reply:
Based on reviewer's valuable guidance, we further evaluated the performance of nhexane for DP2hCOM. As shown in Fig. R18, n-hexane, with the lowest viscosity of 2.97  9. Minor corrections. Keep the format of chemicals purity consistent, such as 98.0%, and 98%.

Reply:
Thanks for the reviewer's valuable guidance. We have carefully checked and corrected the format of chemicals purity in the manuscript and supplementary information.

Manuscript:
▪ Section of preparation and characterization of DPCOMs, Paragraph 2 "The heterocrystalline COM, denoted as DPCOM, was fabricated by dark reaction 36 first in the same way as DCOM, followed by photo reaction under 9.0-mW cm −2 irradiation (Fig.1)." ▪ Section of organics separation performance of DPCOMs, Fig. 4 "The PCOM here is fabricated under 9.0-mW cm −2 irradiation."